1. Field of the Invention
The present invention generally relates to a control circuit for a cathode ray tube, and more specifically to a circuit for generating a grid voltage for the cathode ray tube, which can be reduced to an extremely negative voltage level at the CRT power-off moment, thereby cutting off electron beams immediately and preventing phosphors on the screen from being damaged.
2. Description of the Related Art
Cathode ray tube display devices (CRT display) are one of widely used image display devices, such as monochrome or color televisions, monochrome or color computer display devices, and so on. The CRT display device comprises a cathode ray tube and a peripheral circuit. The function of the CRT is achieved by controlling the path of electron beams by the peripheral circuit to strike the phosphor-coated face of the CRT to radiate light.
FIG. 1 (Prior Art) is a cross-sectional view of a conventional CRT. Referring to FIG. 1, the CRT comprises a front-end portion for producing and accelerating the electron beam (including filament 10, cathode 12, control grid 13, screen grid 14, focusing grid 15 and accelerating anode 16 and deflection jokes 18), a middle portion for deflecting and accelerating the electron beam (including cavity 20 and anode button 22) and a palate portion for practically emitting light (including aluminum film 30, phosphor film 32 and screen 34). Filament 10 is heated by a filament voltage source (not shown) to produce hot electrons, which are then emitted through cathode 12. Generally, the filament voltage is about 6.3 volts. Control grid 13, screen grid 14, focusing grid 15 and accelerating anode 16, using the electrical focusing mechanism, converge the hot electrons emitted from cathode 12 to be electron beam 40. In addition, deflection yokes 18 using the generated electromagnetic field varies the direction of electron beam 40. Practically, deflection yokes 18 include a set of horizontal deflection yokes and a set of vertical deflection yokes. The two sets of deflection yokes, in response to a horizontal scanning synchronizing signal and a vertical scanning synchronizing signal, control the moving direction of electron beam 40, thereby making electron beam 40 to continuously scan the screen. Finally, electron beam 40, which is accelerated and deflected at a scanning angle .theta., move straightly in cavity 20 of the CRT, until striking phosphor film 32 of the palate portion. Electron beam 40 is still influenced by the voltage on anode button 22 during the period of the movement in cavity 20.
In the above CRT, control grid 13 is used to control the number of electrons emitted from cathode 12, thereby adjusting the brightness of the displayed image. In the aspect of structure, control grid 13 like a cylindrical cap encloses cathode 13. In addition, there is a pinhole located at the tip of this cylindrical cap, serving as the passage for the moving hot electrons. Generally speaking, the voltage of control grid 13 is denoted by G1. Control grid voltage G1 has a negative polarity compared with the voltage of cathode 12 and is used for controlling the charge distribution on cathode 12. In more details, when control grid voltage G1 is more negative than the normal value, the number of the electrons emitted from cathode 12 decreases and the brightness of the displayed image is reduced. When control grid voltage G1 is more positive than the normal value, the number of the electrons emitted from cathode 12 increases and the brightness of the displayed image is enhanced. Therefore, control grid voltage G1 can be used to adjust the image brightness.
A problem may occur at the moment of CRT power-off. During the normal operation of the CRT display, electron beam 40 recursively and continuously strikes phosphors film 32 of the palate portion under the control of deflection yokes 18. More specifically, electron beam 40 sequentially scans every horizontal scanning lines on phosphor film 32 in the vertical direction. Therefore, every regions of phosphor film 32 is not continuously stimulated. When the CRT display is powered off, the voltage in the anode cannot immediately disappear due to the capacitance effect. Therefore, electron beam 40 is still emitted within a quite short period after the CRT is powered off. However, the scanning circuitry has already stopped working at the time. As a result, electron beam 40 may constantly strike a special region of the phosphor film 32 during this afterglow period. It is possible that phosphors in this special regions may be destroyed due to the constant striking. Therefore, the issue of the present invention is how to quickly cut off the electron beam at the CRT power-off moment, thereby protect the CRT screen.
The present invention adopts the above grid control voltage G1 to cut off the electron beam when the CRT is powered off. Furthermore, since the above-mentioned problem occurs at the moment of CRT power-off, control grid voltage G1 should be quickly driven to a voltage level to cut off the electron beam immediately.